1. Field of the Invention
The present invention relates to an erasable optical data storage disk to and from which to write, erase and read data.
2. Description of the Prior Art
There have existed morphologically erasable optical disks which utilize an elastomer layer containing organic pigments for data storage, erasure and retrieval. One such optical disk is disclosed in U.S. Pat. No. 4,719,615. This disk, as illustrated in FIG. 1, comprises an elastomer layer 11 on a substrate 10, and a retention layer 12 formed on top of the layer 11 and having a glass transition temperature. On the retention layer 12 is a gap 13 which is topped by a transparent protective substrate 15. Another erasable optical disk of a similar type, as depicted in FIG. 2, has the substrate 10 on which is the elastomer layer 11 which is topped by the retention layer 12 having the glass transition temperature. On top of the retention layer 12 is the transparent protective substrate 15 having pre-grooves or pre-addresses P with a compression layer 14 therebetween. The pre-grooves or pre-addresses are infinitesimal bumps and depressions which are called pits. With this optical disk, the retention layer 12 comprises an epoxy resin containing a red orange pigment. The elastomer layer 11 has a silicon rubber containing a carbon black pigment.
The recording medium that uses these organic pigments utilizes a mechanical deformation of the elastomer layer 11 and the retention layer 12 for data storage, the deformation being a morphological change of the layers due to a temperature change. This recording medium produces pits by having different pigments in different layers absorb laser beams of different wavelengths for selective heating of the material.
In operation, the optical disk works as follows. For writing data, as shown in FIG. 3A, a laser beam of a long wavelength is applied to the retention layer 12, the laser beam being absorbable only by the expandable elastomer layer 11. The beam thermally expands the elastomer layer 11 in the form of a spot. The partially expanded elastomer layer 11a partially raises the retention layer 12, as shown in FIG. 3B. As a result, the retention layer 12 forms a viscoelastically stable projection Pr, as depicted in FIG. 3C.
For reading data, as illustrated in FIG. 3D, a write laser beam is focused onto the target projection Pr. The resulting diffusion of the light by the projection Pr on the deformed retention layer 12 is detected by suitable means as a unit of data.
For data erasure, as depicted in FIG. 3E, a laser beam of a short wavelength is applied to the retention layer 12, the laser beam being absorbable by the layer 12 alone. The heating caused by the laser beam reduces the viscoelasticity coefficient of a projection vicinity layer 12a of the retention layer 12. This causes the expanded elastomer layer 11 to lower the projection Pr on the retention layer 12. Thus the original flat, smooth surface is restored on the optical disk.
One problem with this type of optical disk is that because the reflection factor of the retention layer 12 is low, the projection Pr formed thereon is difficult to recognize. It is thus difficult to recognize pre-grooves or pre-addresses on the protective substrate 15. To bypass this problem, a reflective film has been provided on the disk, which has complicated the disk construction.